Statutory Surplus and Dividend Restrictions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	$ 94.7
Surplus for guaranty	473.7	429.2
Statutory net income (losses)	43.0	10.5	(4.2)
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	2.8
Guaranty recorded surplus	27.2	39.7
Maximum | Underwriter entities
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	$ 15.0